Non-contact web tension meter

ABSTRACT

A web tension meter, particularly suited for measuring the tension in moving webs, e.g. paper in paper machines or a winding machine. The meter is based on the principle of supplying pressurized air through a passage (4) and a passage (11) to an outwardly open space (8) which is surrounded by a measuring opening in the form of an annular slot (9) coupled to a pressure gauge (via 5). The paper is allowed to pass in front of the openings (8, 9) at some millimeters distance therefrom. The measured pressure is quite surprisingly a linear, or near linear, function of the tension in the web.

FIELD OF THE INVENTION

The invention relates to a meter for measuring the tension in movingwebs.

PRIOR ART

The non-contact measurement of moving webs is known in the art and hasbeen employed, for example, for regulating tension when winding paperonto reels and unwinding paper therefrom. Examples of this technique arefound in U.S. Pat. No. 2,945,637. In accordance there with, the web isdrawn in a curved path over a perforated curved plate through the holesof which air is blown so as to produce a kind of air-cushion bearingbetween the plate and the paper. The greater the tension in the paper,the less air is consumed, so that the air pressure increases inwardly ofthe perforated plate. A similar arrangement is known from the SwedishPat. No. 207 513, although in this case the pressure is measured via oneor more measuring apertures located in the wall in which the air-exhaustorifices are seated, so that it is possible to measure the web tensionat a number of positions across the width of the web.

These known non-contact web-tension measuring apparatus, however, havethe disadvantage that it is necessary to change the direction of theweb, by drawing the same over a plate or like element, admittedly whilesupporting the same on an air cushion and therewith in a manner which iscontactless at least in principle, but not reliably so in practice.Although, as far as the arrangement disclosed in the aforesaid Swedishpatent is concerned, it is possible to measure web tension at mutuallydifferent locations across the width of the web, these measurements canonly be taken at fixed locations determined by the construction.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a non-contact web tensionmeter which can be placed against a web without needing to change thedirection of the web, and the position of which, can be freely moved toany part of the web whatsoever across the breadth of the web, and whichis therewith suitable for incorporation in a transversable measuringhead, for co-action with other measuring devices operative in measuringthickness, moisture content and other variables of interest with regardto process controls and quality controls. The measuring process shallpreferably provide a linear or substantially linear result.

In explanation of this object, it may be mentioned that during his workwith thickness gauges for measuring the thickness of moving paper webs,the present inventor discovered that the thickness of a paper web, atleast when measured in a contactless manner or with low surfacepressure, surprisingly changes when a change in web tension occurs. Onemight presume that an increase in tension would result in a thinnerpaper, due to transverse contraction. What was found duringexperimentation, however, was something far more complicated, a matterwhich, moreover, is contingent on the kind of thickness gauge or meterused. When a non-contact meter of the kind disclosed in SwedishPublished Application No. 434 997 is used, stretching of the web willinitially result, in the case of newsprint, in the measured thicknessincreasing by 3-4 micron (force 0-0.2 N/m) and then increasing moregradually by some further microns (force 0.2-1.0 N/m), and only whenthis point is reached, decreasing when tension is increased. This effectis apparently due to the "pile" of the paper rising as the paper isplaced under tension. Consequently, in order to obtain a true value inrespect of the thickness of the paper, it is necessary to know thetension therein, in order to make a suitable correction. Newsprint canhave a standard thickness of 80 microns, and it is desirable to be ableto maintain control of this thickness to within about one or twomicrons, or better. In addition, it should be noted that contactingthickness meters have an even greater dependency on web tension than dopresent day non-contacting thickness meters.

Measurements of the web tension, correlated with other measurements, isdesirable in continuous manufacture control processes, for controlling apaper machine. This control is normally effected by heating a rollerlocated in the machine to varying degrees of temperature at differentlocations along the length of the roller, to achieve a uniformthickness. A web of uniform thickness is a requirement from theprinters. Data relating to the web tension is also important when thepaper is to be wound on reels, and there is reason to suspect thatirregularities in the tension of the wound web are the cause in manycases of unforeseen web fractures when, for example, printing newsprint.A paper roll which has been wound properly with respect to web tensionis also less likely to become nonround.

The aforementioned objects and advantages are achieved in accordancewith the invention by means of a non-contact web tension meter of atelescopic construction controlled by pressurized gas, such as to bewithdrawn when pressure gas falls away. In a preferred embodiment, themeter can be extended telescopically only when the pressure exceeds agiven minimum pressure. In a further improvement in this respect, and animproved function during a measuring operation, the width of thepressure aperture is small in relation to the width of the web andpreferably not greater than one-twentieth thereof, and it is suitable topermit solely the lips which define the pressure aperture and themeasuring aperture, to face towards the web.

BRIEF DESCRIPTION OF THE DRAWINGS

For purposes of illustration, the invention will now be described withreference to an embodiment thereof illustrated in the accompanyingdrawings.

FIG. 1 is a side view of a web tension meter.

FIG. 2 illustrates the same web tension meter in cross-section.

FIG. 3 illustrates a stationary testing station.

FIG. 4 illustrates a test result obtained in the testing station shownin FIG. 3.

FIG. 5 illustrates variations in thickness with web tension in respectof two mutually different papers.

FIG. 6 illustrates the effect of a correction made to a paper profilewithout taking web tension into account.

DESCRIPTION OF PREFERRED EMBODIMENT

FIG. 1 is a schematic side view of a web tension meter according to theinvention. Seated in a holder 3 is a measuring head, which issubstantially of cylindrical shape and has a diameter of 30 mm. Apressurized air inlet is shown at 4, and a pipe 5 is connectable to apressure meter. Such pressure meters are known to the art in manyvariations. In the illustrated embodiment there has been used a lowpressure transducer designated Mod. 261 from Setra Systems, 45 NagokPark, Acton, Mass., USA. As clearly shown in FIG. 2, the measuring headis divided into two parts, a stationary part 1 and a movable part 2,these parts moving telescopically relative to one another. Thestationary, upper part is hollow and has a narrow opening 14 and a wideropening 16 and a cap 18, through which the pressurized-air pipe 4extends. The movable part comprises a nozzle part which is firmlymounted on a stem 15 fitting in the opening 14 and the other end ofwhich carries a piston 13 which fits in the wider opening 16. Acompression spring 12 is mounted so as to tend to draw the stem 15inwardly to the position shown in FIG. 2. An air passage 11 extendsthrough the stem, up to the nozzle part, and is terminated in onedirection in a sealing ring 17, which is located on the upper side ofthe piston and which, in the illustrated position, seals against theinner surface of the cap 18, around the air inlet hole 4.

It will be seen that the piston 13 when occupying the illustratedposition will not be affected by the pressurized air introduced untilthe pressure is of such magnitude as to free the sealing ring 17 fromthe cap 18, against the action of the spring 12.

The passage 11 opens into the outwardly open space 8, via a coarsesintered plate 19. The sintered plate contributes in holding up thepressure in the passage 11, so that sufficient pressure is obtainedthrough the supply of pressurized air for pressure to be exerted overthe whole of the upper surface of the piston 13, whereafter the pistonforces the movable part 2 out to a working position, (not shown).

When using the meter to measure the tension in a web, the web is broughtinto position for abutment and, in use, is spaced at a distance ofapproximately 1 mm from the lower portion of the movable part 2 andforms a defining wall of the space 8. Extending around the space 8 is anannular chamber 7 with an annular measuring slot 9, outwardly defined bya lip 10. When the space 8 is pressurized to a pressure of 0.5-1 bar,the yielding web will move away to a greater or lesser extent, dependingupon the tension prevailing in the web. The pressure measured in thepressure gauge connected to the measuring slot 9 through the passage 5will vary with web tension. The measuring result, on the other hand, isnot particularly influenced by the pressure of the input air, and hencequite primitive pressure control devices will suffice.

It might be possible to guess beforehand that such a dependency is to beexpected, by relatively elementary considerations. However, the factthat this effect is found to be linearly, or substantially linearly,dependent on the web tension, rendering the invention well suited forthe aforesaid measuring function, is likely to surprise one of ordinaryskill in this art.

This linearity has been determined experimentally. In one experiment astrip of paper 30 (type newsprint) having a length of 2 m and a width of440 mm was hung vertically between two supports 31 and 32, and wasloaded with a container 33 into which water could be poured. With thecontainer empty, the tension corresponded to a load of 220 grams. A webtension meter 34 was placed against the surface of the paper. Thegeometry will be seen from FIG. 3, in which the measurements are inmillimeters. The result obtained with this arrangement is shown in FIG.4, where the values in percentage of a full-scale reading on thepressure meter used are plotted on the X-axis. The curve obtained waspractically linear.

It will be understood that the force conditions are not entirely simple,since the arrangement functions with a flowing gas. It will be perceivedthat there prevails in the space 8 (FIG. 2) a static pressure which,quite naturally, will force the paper outwardly, away from the measuringhead. The air, however, simultaneously, laterally in the gap formedbetween the head and the paper web, creating a vacuum force inaccordance with Bernoulli's law, which strives to draw the paper towardsthe head. The measured pressure is influenced by this subpressure. Thatthe result of these different effects brings about a result which is alinear, or substantially linear function of the web tension, is apt tocause surprise, as is also the fact that the measuring result depends solittle on the applied pressure, within wide limits.

Tests have also been carried out with moving webs, with paper wound fromone roll to another. As far as could be determined, the measuring resultis not influenced by the speed at which the web moves. In thisarrangement, the web forms, at a location opposite the head, a littlepronounced "valley", having a length in the feed direction of sometwenty centimeters and a width which slightly exceeds the width of thehead.

In order to illustrate the particular problem within the paper industrywhere the invention affords a solution, reference is made to FIG. 5,which shows the change in paper thickness in microns as a function ofweb tension. The curves 51 and 52 show the results obtained with testscarried out on newsprint obtained from two different Swedish papermills. The difference is not one of chance; on the contrary, papersamples obtained from the two mills consistently each exhibit itsparticular curve form. It will be seen that; when stretching, there is arapid increase in thickness succeeded by a more gradual increase inthickness with a further increase in the tension, finally followed by adecrease in thickness for web tensions above about 1 N/m. It is apparentthat no accurate measurement of the thickness of paper webs is possiblewhen they are tensioned, unless the effect of the web tension iscorrected for.

FIG. 6 illustrates the effect of a thickness measurement which isimpaired by errors due to failure to compensate for web tension. FIG. 6shows along the X-axis the width of a paper web which is assumed to beinitially of uniform thickness, i.e., which does not deviate from adesired standard thickness. This standard uniform thickness is shown bythe line 61. A variation in web tension exists, however, across thewidth of the web, shown by the broken line curve 62, which is quiterepresentative, since web tension is often lower at the edges of theweb. If the thickness of the web is now measured, the value obtainedwill be in error, the paper being found thicker in the center of theweb. When correcting this supposed error until a uniform thickness isfound in the thickness meter, there will actually be obtained amanufactured paper having a true thickness curve which exhibits thedeviation shown by the chain line 63.

As to the accuracy aspect of thickness measurements, the following canbe said. Standard newsprint has a thickness of roughly 80 microns.Newsprint which exhibits variations in thickness in the order of 10microns is considered to be second-rate, and a standard manufacturingtolerance is at present 2 microns. Variations in thickness aresignificant with respect to both the quality of the print obtained andto the risk of web fracture. High accuracy measurement of web thicknessduring manufacture would be highly beneficial in producing a unifom dryweight of the paper across the whole of its breadth. At present, it isnecessary, instead, to make corrections by drying the web to varyingdegrees at different locations across the width, so as in this way toreduce the variations in thickness when calendering the formed paper.The varying magnitudes of moisture content of the finished paper rollsare economically disadvantageous to the paper mill, which, by applyingbetter controls--solely possible by improved thicknessmeasurement--could reduce the consumption of raw materials for each tonof paper produced without loss of quality. Winding of newsprint intorolls (diameter 2 m) can also be controlled in an improved manner, and asuitable tension distribution obtained, all by accurate measurement ofthe tension of the web. There is a well-founded general suspicion thatpoor lateral distribution of the web tension profile is an essentialsource of web fracture in printing shops.

What is claimed is:
 1. A non-contact web tension meter for measuring thetension of a web of material having a breadth and drawn under tensionbetween two guide means, said meter incorporating means for supplying apressurized gas to a space therein which is open in one direction andwhich is delimited by one side of said web placed in close proximity tosaid space, and a pressure gauge adapted to measure the gas pressurenear said web at a side of said space, said meter being freely movableacross the breadth of said web, and comprising a holder (3), having ameasuring head attached to said holder and incorporating apressurized-gas inlet (4), a gas passage (11) connected to said inletand extending to said space (8), and an annular chamber (7) surroundingsaid space (8), and having an annular opening (9), said space (8) andsaid opening (9) terminating adjacent an end plane in said measuringhead, and said annular chamber being connected to a pressure sensor. 2.A non-contact web tension meter according to claim 1, wherein saidmeasuring head has first and second parts which are telescopicallymovable in relation to one another, said first part (1) being fixed tosaid holder (3) and carrying said pressurized-gas inlet (4), and saidsecond part (2) presenting said open space (8), which is intended to beplaced under pressure, and said annular chamber (7), said first andsecond parts functioning as a piston and a cylinder, respectively, formoving the same mutually apart, against the action of a spring (12),upon introduction of said pressurized gas.
 3. A non-contact web tensionmeter according to claim 1, comprising a stem (15) mounted on an end ofsaid second part (2) remote from said end plane of said measuring head,said gas passage (11) extending through said stem and terminating with apiston (13) whose diameter is grater than that of said stem, said firstpart (1) having a first, elongated opening (14) for accommodating saidstem, a second opening (16) being located in an extension of the stem(15) and being adapted to accommodate said piston (13), said spring (12)being a compression spring arranged in the second opening between saidpiston (13) and said first opening (14), said gas passage (11)terminating at the end of said piston (13) opposite said spring (12)with an annular seal (17) in sealing abutment with said pressurized-gasinlet (4), an exit orifice of which is located in a wall (18) definingsaid second opening.
 4. A non-contact web tension meter according toclaim 3, comprising a throttling plate (19) arrangement between saidpassage (11) and said space (8) to be pressurized.
 5. A non-contact webtension meter according to claim 4, wherein said throttling plate (19)is a sintered plate.
 6. A non-contact web tension meter according toclaim 1, wherein the opening of said space (8) to be pressurized isarranged to take up less than one-twentieth of the width of the web tobe measured, and is defined by a first lip (6) towards said annularopening (9), which in turn is defined outwardly by a second lip (10),said first and second lips jointly defining said annular opening (9),that side of said measuring head (2) facing said web solely comprisingthe surface formed by said two lips.